A gas turbine engine includes a fuel-oil heat exchange system in which the fuel is continuously heated by a primary hot oil flow in a primary fuel-oil heat exchanger and the fuel is selectively heated in a secondary fuel-oil heat exchanger by a secondary hot oil flow selectively passing through or bypassing a secondary fuel-oil heat exchanger, controlled by a thermal valve. The bladder is adapted to be inflated by a fluid. One or more current sources may be used to stress the state-monitoring memory element. Movement of the chip-carrier plates is controlled such that the selected chip-carrier plate is positioned at a collection point to be collected from the magazine. The heat source may be combustion of a fuel, solar, geothermal, chemical reaction, or waste heat in the form of a hot gas.